This invention relates to a thermal printing head for use in a device which is adapted to print information on thermosensitive paper by means of thermal energy.
Various types of thermosensitive papers which produce colors in response to the application of thermal energy are now available on the market. Thermal printing heads of the kind in which heat is generated by the concentrated flow of current through a resistor portion have been developed and are now available on the market. Thermal printing heads of this kind can be broadly classified into three types the silicon planar type, the thin film type and the thick film type. Among the three types of thermal printing heads, this invention particularly pertains to a thermal printing head of the silicon planar type.
Conventional thermal printing heads of the silicon planar type have had a plurality of heat-generating elements arranged in a matrix form or in a line, which heat-generating elements are separated from each other by means of moats provided therearound so as to avoid blurring of printing patterns due to heat leaking between the neighboring elements thereby forming a plurality of mesas which are separated by air isolating space therebetween.
However, with a thermal head of this kind, there has been a problem that during repeated heating and cooling cycles over a long period of time, fragments of thermosensitive coloring materials and fibers of paper gradually accumulate in the moats between mesas, and finally adhere on the surfaces of mesas, and thus prevent the contact of the mesa surfaces and the thermosensitive paper, with the result that not only does the intensity of the printed patterns become less but also the temperature of the mesa surfaces becomes excessively high causing shortening of the life of the thermal printing head.
Furthermore, there has been a problem that when the mesa surfaces are in contact with the thermosensitive paper, mesa edges are subjected to stress concentration and during repeated contact of the mesa surfaces and the thermosensitive paper, the mesa edges are broken or the mesas themselves are exfoliated from the ceramic substrate.
In addition to the above-described problems, there has been a problem that since the thickness of the silicon chip forming the thermal printing head is thin being on the order of about 50 m, the silicon wafer in which is formed the mesa moats about 30 m depth is easy to crack, so that it is almost impossible to form mesa moats on the wafer alone, for the reason that the mesa grooves are formed on the silicon chip which is bonded to and electrically insulating substrate such as ceramic whereby it is not suitable for mass production.
The problems described above could be solved if the head surface to be brought into contact with the thermosensitive paper was flat and had no moats; however, with such a structure, the printing pattern is blurred due to heat leaking between neighboring heat-generating elements.